1. Field of the Invention
The invention relates inter alia to a method and a system for producing clean hot flue gas with a low content of volatile organic compounds (VOC's), NOx and dust, and clean ash with a low carbon content by means of a stage-divided thermal reactor. In the stage-divided thermal reactor the conversion process of the solid fuel is in separate vertical stages (from below and up): ash burn-out, char oxidation and gasification, pyrolysis, drying and a gas combustion stage wherein gas from the gasifier is combusted. The gas-combustion stage functions both as gas burner and heat source for drying and pyrolysis the top layer of the updraft gasifier.
2. Description of Related Art
Production of hot flue gases during thermal conversion of fuel is well known. Hot flue gas can be used for several purposes, e.g. for production of steam, hot water, hot oils etc.
Reference is made to WO 2007/036236 A1, which concerns heat recovery of hot gas produced in a thermal reactor, by injecting water into the gas at one or more injection zones in such an amount and in such a way that, due to water evaporation, the gas temperature is reduced to below 400° C. and the gas dew point becomes at least 60° C., preferably at least 70° C., possibly 80 or 85° C. The gas can then be led through a condensing heat exchanger unit, wherein at least some of contents of water vapour in the gas are condensed, and the condensing heat can be utilized for heating of a stream of fluid, mainly water
and
WO/2007/081296, which relates to a gasifier that can run by downdraft or updraft to produce syngas from sorted/unsorted municipal solid waste (MSW), refuse-derived fuel (RDF), industrial waste including sludge from waste water treatment plant, leather industry residues, agricultural waste and biomass comprising: a bottleneck zone, a drying zone, a pyrolysis zone, a reduction and oxidation zone, an ash section, a safety valve, a rotary valve, a vibrator and several igniters and
U.S. Pat. No. 6,024,032 A: A process for the production of heat energy from solid carbonaceous fuels is disclosed. The process comprises subjecting the carbonaceous fuel to substantially anaerobic pyrolysis in at least one first zone and thereafter transferring the char resulting from the pyrolysis to a second zone which is separated from the first zone or zones. The char is subjected to gasification in the second zone by introduction of primary combustion air, optionally with steam and/or recycled exhaust gas. The off gases from the second zone and the pyrolysis gases from the first zone or zones are thereafter subjected to secondary combustion. The first zone or zones is heated by heat derived from the secondary combustion. Ash is removed from the bottom of the second zone
and
WO 01/68789 A1: A staged gasification process and system for thermal gasification of special waste fractions and biomass, e.g. wood, comprising a drier (1), in which the fuel is dried upon contact with superheated steam. The dried fuel is fed into a pyrolysis unit (4) to which superheated steam is also supplied. The volatile tar, containing components produced in thepyrolysis unit (4), is passed to an oxidation zone (5) in which an oxidation agent is added so as to cause a partial oxidation, whereby the content of tar is substantially reduced. The solid fuel components from the pyrolysis unit (4) may be fed into a gasification unit (6) to which hot gases from the oxidation zone (5) are supplied. In the gasification unit (6) the solid fuel components are gasified or converted to carbon. The gas produced in the gasification unit (6) may be burnt in a combustion unit (7), such as a combustion engine. Thus, a gasification process is obtained for gasification of biomass and waste with a high energy efficiency, low tar content of the gasification gas and with moderate risk of slagging for a wide spectrum of fuels, including fuels with a large content of moisture
and
WO 2008/004070 A1: A method of controlling an apparatus for generating electric power and apparatus for use in said method, the apparatus comprising: a gasifier for biomass material, such as waste, wood chips, straw, etc. . . . . Said gasifier being of the shaft and updraft fixed bed type, which from the top is charged with the raw material for gasification and into the bottom of which gasifying agent is introduced, and a gas engine driving an electrical generator for producing electrical power, said gas engine being driven by the fuel gas from the gasifier. By supplying the produced fuel gas directly from the gasifier to the gas engine and controlling the production of the fuel gas in the gasifier in order to maintain a constant electrical output power, the necessity of using a gas holder between the gasifier and the gas engine is avoided.
Thermal Reactors
Solid fuel is usually converted into a burnable gas (gasification) or into a flue gas (combustion) in a moving bed or a fluid-bed reactor.
Moving-bed reactors are typically divided into following categories: updraft (air/gas goes up and fuel down); downdraft (air and fuel go down) or grate/stoker-based system (moving grate, vibrating grate, stoker) where fuel moves horizontally (often with a slope downwards).
Fluid-bed reactors are typically divided into the following categories: bubbling fluid bed (BFB), circulating fluid bed (CFB) or entrained flow (EF).
Most reactors are originally designed for conversion of coal. Fresh solid fuel such as biomass or waste has very different properties compared to coal. Especially the content of volatiles and water is much higher in biomass and waste. In coal, the volatile content is normally below 30%, whereas for biomass and waste the volatile content is normally above 65% (dry ash free weight basis). Further, the content of water in fresh biomass and waste is often above 20%, and even often above 50%, so drying of the fuel is often a very important issue in biomass and waste reactors. Further, the content and the composition of the ash can be very different for coal and biomass/waste. Also the content of alkali metals (Na, Ka), Chlorine, Potassium, Silica etc. may be much higher, and ash melting points of biomass and waste are known to be much lower than in coal.
Therefore, standard “coal reactors” are not optimal for conversion of biomass and waste.
Feeding Systems and Means of Transporting the Fuel
Feeding systems are normally screw or push type or pneumatic “spreader stoker” feeders.
In grate systems, the fuel is transported by the grate. In most cases, combustion air is led through the grate. These systems may have several problems including hot spots on the grate, uneven air distributions, ash/char falling through the grate, controlling the stages on the grate etc.
In fluid-bed systems, the fuel is mixed with the bed material. The fluid-bed systems may have problems with separating the bed material from the ash, and with separating the different process steps as fluid beds are normally well-stirred reactors.
Updraft gasifiers are usually used when the aim of the conversion is production of a burnable gas. Updraft gasifiers are usually used for production of town gas and lately also for gas-engine operation, such as described in WO 2008/004070 A1. In updraft gasifiers, there is a simple feeding and transporting mechanism, both into the reactor and out of the reactor, where the ash can be removed in a cold state. When the ash layer is in the bottom of the reactor, the gasification agent (air/steam) is added. It is well known that updraft gasifiers convert the fuel very well and that there is very little carbon in the ash. Updraft gasification technology is known as a simple and robust technology. However, the updraft gasification technology has some disadvantages such as                The produced gas has a high content of CO, tars and other unburned gases, which are difficult to clean up when syn-gas production is the aim of the gasifier        Scaling up is normally difficult as round shapes are usually used        
The bed is relatively high, such as 4 meters or more when wet fuel is to be used as drying needs long reaction time. In systems such as U.S. Pat. No. 6,024,032 A and WO 01/68789 A1 one or several of the process reactions are physically separated from the others. This can have some process advantages, but it also has the disadvantages that the reactors become                Larger        More expensive to built        More expensive to do maintenance on.Water Content in the Fuel        
Normally, a combustion unit is made for either fuel with high water content (and low heating value) or for fuel with low water content (with high heating value). However, costumers often prefer a unit that can burn a broad range of fuels.
WO 2007/036236 A1 describe a solution to this problem: If the combustion unit is designed for wet fuels and receives a dry fuel then the lack of water in the fuel can be compensated by adding water to the fuel or into the thermal reactor, so the drying zone doesn't become too hot, thus resulting in NOx formation and/or overheating materials.
Gas Combustion
One of the major technical and environmental problems in converting solid fuels into a clean flue gas is to prevent unwanted substances in the flue gas. These substances include                Organics materials: CO, PAH (Poly Aromatic Hydrocarbons), Dioxin or VOC        NOx        Particles        Other.        
In state-of-the-art combustion plants, there are often several air-injection stages (primary, secondary and tertiary air) with a high number of nozzles and/or downstream gas cleaning means such as NOx removal filters, oxidizers for organic materials or dust collectors to get low enough emissions.
Combustion of organics materials can be optimized, by ensuring effective mixing between oxygen and gas; and ensuring high retention time, such as 2 seconds or above, and by ensuring a high combustion temperature, such as 900° C. or above.
Thermal NOx is formed in the gas combustion stage and is mainly depending on the temperature. The higher the temperature is, the more NOx formation, but also the higher the oxygen content is, the more NOx is formed. The NOx formation is moderate when the temperature is below 1100° C., but NOx formation accelerates when the temperature gets much above 1100° C.
Particle emission is normally high from fluid-bed reactors and for grate systems whereas updraft gasifiers are known to produce a gas with very few particles.
Fuel NOx
Besides thermal NOx, as described above, NOx can be formed from the nitrogen in the fuel: Fuel-NOx is formed when there are over stoichiometric air-fuel ratio in the fuel. This is often the case in grate systems and in fluid bed combustors, whereas in updraft gasifiers this it not the case. It is well known that updraft gasifiers produce gas with low NOx.
Oxygen Content in Flue Gas
An important parameter for combustion plants is the oxygen content in the flue gas. The lower the oxygen content is, the better.
There are several advantages to low-excess oxygen including:                Lower investment cost and energy consumption for air blower        Lower amount of flue gas and therefore smaller and cheaper components        downstream of the thermal reactor        Higher steam ratio in the flue gas and therefore better radiation properties        Higher water dew point in the flue gas and therefore higher energy efficiency in a condensation cooler.        
Typically, the excess air is more than 5%, such as 7% (dry basis), which corresponds to a lambda (stoichiometric ratio) of 1.3 or more.
Steam Content in Flue Gas
There are several advantages of a high steam content in the flue gas. These advantages include, but are not limited to:                Radiation properties improved        Recovering of heat in condensing unit improved        Soot formation prevented        Limitation of temperature and hence NOx formation.Air Distribution        
In typical combustion plants, air is distributed to many of the combustion stages:                The drying stage        The pyrolysis stage        The gasification/oxidation stage        The ash burn-out stage        The gas combustion stage, and here often in several stages (secondary and tertiary stages).        
When oxygen is let into the drying and/or pyrolysis stage and/or oxidation stage it is not specifically aimed for either burn-out of de-volatilized char or gas combustion, which then results in a high level of excess air for the total plant.
Steam and Oxygen Content in the Combustion Air
Normally untreated air is used for combustion, but the properties of the air can be improved by adding steam and/or oxygen to the air.
Steam in the primary air results in lower temperatures in the oxidation zone, which prevent slagging of the ash and it improves the gasification reactions (H20+C→CO+H2).
Steam in the secondary air reduces temperatures in the gas combustion section, thus reducing NOx. Further steam prevents soot formation.
A high content of oxygen results in a lower mass flow of combustion fluid, thus reducing size of plant.
Carbon Content in the Ash
In grate and fluid-bed systems, the carbon content of the ash is often 10% or more. This leads to an efficiency and environmental problem: The carbon contains valuable energy, which is not utilised, but also environmental unfriendly substances, such as PAH.
Further, a main technical problem is often that ash sinters at 700-900° C. depending on the ash components. To prevent ash sintering in fluid beds and grate systems, the char content is often high, such as 10% or above.
Further, in grate systems, unburned fuel with high char content often falls through the grate; hence the char content in the bottom ash will increase.
Ash Removal System
In grate systems and in fluid-bed systems, the ash removal systems are costly and complicated.
In fluid-bed systems, ash and sand are mixed, so after ash/sand removal, the sand needs to be separated from the ash.
Ash-removal systems of updraft gasifiers can be made simple, as the temperature on the grate is low.
Moving Parts in the Reactor and in the Hot Stages
In grate systems, the fuel is moved from the inlet to the ash outlet by a grate. Typically, this grate is made of high-grade steel, which is both costly and also needs replacement. Normally, a part of the grate is replaced at least every year, and costs related to downtime, materials and labour may be very high.
In some updraft gasifiers, there is a large stirrer in the top to even out the fuel.
Shapes
Fluid-bed reactors and updraft gasifiers are typically round, whereas grate systems are typically rectangular.
The round shape in typical updraft gasifiers results in a maximum size of about 10 MWthermal. A typical key figure of updraft gasifiers is 1 MW/m2 of char gasification reactor. At 7 MW, the diameter is then more than 3 m, and at this size, the flow may become uneven. Therefore, it is recognized that app. 10M W is the maximum input of updraft gasifiers.
Size of Plants
Combustion plants are made in very small scale, such as stoves of 5 kW and even below, or in very large scale, such as coal-fired power plants, which can be several hundred MW.
Turn-Down Ratio
A typical turn-down ratio of grate systems and fluid beds is about 1:2, whereas updraft gasifiers may have a turn-down ratio of 1:10 or even 1:20.